1. Field of the Invention
The invention relates generally to the field of seismic surveys conducted between wellbores drilled into the Earth's subsurface. More specifically, the invention relates to seismic energy sources for use in wellbores having a pipe or casing set therein.
2. Description of the Related Art
Oil and gas are typically obtained from the Earth's subsurface from wells drilled through subsurface rock formations. Oil and gas wells can range in depth from a few hundred feet to over 20,000 feet. In general, a well is made by drilling a hole in the Earth called a “wellbore.” Sections of metal pipe connected together end-to-end form a “casing” that is positioned in the wellbore after drilling is completed. Once the casing is installed, cement is typically pumped down the casing and out through the bottom of the casing to fill the annular space between an inner surface of the wellbore and an outer surface of the casing. The cement exits the bottom of the casing and flows into the annular space between the wellbore and the casing as it moves back up toward the surface. When the cement hardens it forms a sealing bond between the inner surface of the wellbore and the outer surface of the casing. This sealing bond serves many purposes, including protecting oil and gas in reservoirs below the surface from contamination. Within the well, perforations are formed in the casing adjacent a rock formation containing oil and/or gas (a “reservoir”). These perforations allow the oil and/or gas to enter the casing from the reservoir. Once inside the casing, the oil and/or gas may be transported to the surface through a tubing that is inserted into the casing to a selected depth.
It is often desirable to obtain information about formations in the Earth surrounding a well. Such information may be used, for example, to target areas within underground formations most likely to produce oil and/or gas, thereby improving well production and reducing operating costs. One way to obtain information about formations in the earth surrounding a well is to use a source to generate seismic waves that pass through the geologic formations adjacent to the wellbore, and a receiver that receives at least a portion of the seismic energy. Transmission factors evidenced by the amount of time it takes the signal to travel from the source to the receiver, and/or the amplitude or phase of the received signal compared to that of the transmitted signal, are generally indicative of formations surrounding the wellbore. Such investigatory techniques are generally called “seismic” techniques.
Seismic evaluation methods known in the art include three-dimensional vertical seismic profiling (3-D VSP) and crosswell seismic tomography, and by using such techniques high resolution images regarding underground formations and fluids within underground reservoirs can be obtained. A vertical seismic profile (VSP) is typically acquired using at least one seismic signal source located on the Earth's surface near a wellbore and a receiver deployed in the wellbore. A 3-D VSP is typically acquired using a source triggered at multiple positions on the surface about a wellbore and multiple receivers positioned at different depths within the wellbore. A “reverse” 3-D VSP technique typically involves positioning multiple receivers on the surface about a wellbore and triggering a seismic source at one or more locations within the wellbore.
Crosswell seismic techniques generally include deploying a seismic source in one wellbore and a seismic receiver in another wellbore. Crosswell seismic techniques can generally provide higher resolution data than is possible with other techniques that deploy a source and/or receiver on the surface. Data acquired using crosswell techniques are advantageous in that the data can be directly referenced in depth, enabling accurate correlation of the data with, for example, conventional “well logging” data.
Currently available seismic signal sources designed for use in wellbores, and thus suitable for crosswell investigations, can generally be categorized as impulsive sources or swept frequency sources. Impulsive sources, such as air guns, sparkers and explosives, typically generate high amplitude signals of short duration (i.e., impulses). Due to their high amplitudes, impulsive sources can damage well casings and cement liners. As a result, impulsive sources are typically subject to minimum depth restrictions when used in wellbores.
Swept frequency sources are generally capable of producing cyclic signals over a range of frequencies, and are typically controlled to produce cyclic signals at frequencies that increase (or decrease) linearly with time. Currently available swept frequency sources generally fall into two categories: fluid-coupled sources and clamped sources. Fluid-coupled sources must be operated in fluid-filled wells, and thus find limited use in gas-filled wells. Fluid-coupled sources also typically produce signals of low amplitude, particularly at lower frequencies, and exhibit undesirable wave propagation in and along the wellbore. Clamped sources typically include mechanical and/or hydraulic clamping systems to securely couple the source to a well casing. Such clamping systems can damage well casings, and generally require complex surface support systems that create reliability problems and make clamping sources expensive to deploy.
While the invention is susceptible to various modifications and alternative forms, specific examples thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.